- structure the Float64 conversion tests the same way as for Float32
- add additional test cases, including one that exposes a current issue
  (currently disabled, same issue as was fixed for Float32)

The Float64 fix will be in a subsequent change for easier reviewing.

Change-Id: I95dc9e8d1f6b6073a98c7bc2289e6d3248fc3420
Reviewed-on: https://go-review.googlesource.com/10351Reviewed-by: Alan Donovan <adonovan@google.com>